In a cellular wireless communication system including a plurality of cells, a wireless transmit/receive unit (WTRU) physically moves from one cell, (i.e., communicating with a particular base station), to another cell to maintain an interruption-free connection. A cell handover is automatically performed by the WTRU through constant evaluation of signal strength of the serving cell, (i.e., the cell with which the WTRU is currently communicating), and various neighbor cells in the vicinity.
When the WTRU is locked onto a serving cell and is attempting to estimate the signal strength of a neighbor cell, the strong signal from the serving cell overpowers the signal transmitted by the neighbor cell and therefore creates measurement noise. This measurement noise may mask out the signal of the neighbor cell to the point that detection of the neighbor cell is not possible. Thus, a problem exists whereby an accurate estimate of the signal strength of the neighbor cell signal cannot be performed.
“Brute-force” detection and estimation using correlators show that if the signal power of the serving cell is approximately 15 dB stronger than the signal power of the neighbor cell signal, (i.e., a 15 dB “power spread”), the neighbor cell cannot be detected due to strong cross-correlation components from the serving cell creating a high measurement noise floor.
In a spread-spectrum cellular communication system, WTRUs typically lock onto a primary serving cell for a communication link, but continuously look for a neighbor cell that may have snore favorable signal characteristics than the primary serving cell. If such a neighbor cell is found, it may be advantageous for the WTRU to perform a handover to the neighbor cell in an attempt to improve connectivity, (as measured by data rate, voice quality, or the like).
The metric typically used to measure signal quality is the received signal code power (RSCP). For example, in a universal mobile telecommunication services (UMTS) time division duplex (TDD) system, each cell transmits a unique beacon channel containing a distinctive 456-chip sequence, (i.e., the “midamble”), orthogonal to adjacent cells. The WTRU has a priori knowledge about the available beacon channels surrounding the serving cell, and can configure an RSCP measurement unit to measure the RSCP of the serving cell and the neighbor cells. After compiling a list of candidate neighbor cells, the WTRU can evaluate the RSCPs of the neighbor cells against the RSCP of the serving cell and determine if a handover to a new cell is desirable.
In its simplest form, an RSCP measurement is the cross-correlation between a known target cell midamble and the received signal. The target cell refers to a cell for which the RSCP measurement is requested. The received signal contains the target cell midamble, other cell midambles and general receiver noise that can be modeled as additive white Gaussian noise.
FIG. 1 shows a cellular system which includes a receiver 105 which receives a midamble m0 broadcast from a transmitter 115 in a serving cell 120, and a midamble m1 125 broadcast from a transmitter 130 in a neighbor, (i.e., target), cell 135. Because the midambles m0 and m1 are designed to be uncorrelated, the receiver 105 can separately detect the existence of the serving cell 120 by detecting the midamble m0, as well as the existence of a neighbor cell 135 by detecting the midamble m1. By computing the correlation of different midambles with the received signal, the receiver 105 detects a peak in the correlation sequence which indicates the position of the midambles, (relative to a sample window), determines the midamble strength, (proportional to the magnitude of the detected peak), and a midamble phase, (relative to a sampling phase of an analog-to-digital converter (ADC) (not shown) in the receiver 105).
FIG. 2 shows a graphical representation of the correlation of a “typical” midamble with itself, otherwise known as the autocorrelation. Each cell has its own unique midamble. The finite length of a midamble is around 456 symbols in length.
FIG. 3 shows the correlation of two different midambles, otherwise known as the cross-correlation of the two midambles, and demonstrates the fact that there is still some small degree of correlation between the midambles.
The challenge in measuring the RSCP of a target cell is that the presence of other midambles in the received signal creates measurement noise. If the signal strength of the target cell is low relative to the measurement noise, the accuracy of the estimate is degraded. Moreover, detection of the target cell may not be possible if it is buried in the noise.
When the receiver attempts to measure the RSCP of a neighbor cell in the presence of a serving cell, the strong signal of the serving cell causes an automatic gain controller (AGC) of the receiver in the WTRU to attenuate the overall received signal. Thus, the neighbor cell signal is also attenuated. If the power spread, (i.e., the difference in received signal power between the serving cell and the neighbor cell), is too high, accurate neighbor cell measurements cannot be performed because the neighbor cell power is too close to the noise floor of the receiver in the WTRU.
FIGS. 4A-4D are graphical representations of correlation of a neighbor cell signal in the presence of the strong signal of the serving cell with power spreads of 0 db (FIG. 4A), 6 db (FIG. 4B), 12 db (FIG. 4C) and 18 db (FIG. 4D) in a conventional receiver. The noise floor seen in FIGS. 4A-4D is due to the cross-correlation between the serving cell and the neighbor cell. As the serving cell received power dominates over the neighbor cell received power, the correlation peak of the neighbor cell approaches the noise floor. At a spread of 18 dB, as shown in FIG. 4D, the RSCP of the neighbor cell cannot be measured because no unique peak exists. Because of the high cross-correlation noise floor created by the strong serving cell power, the presence of the neighbor cell is buried in the noise. Thus, the neighbor cell would not be detected.